Vandals broke in to a sewage treatment plant and released sludge into a river.
They caused £1,000 damage at Anglian Water's sewage works in Great Tey Road, Little Tey, and sludge overflowed from tanks on to the ground.
Alarms were set off at about 5.35pm on Sunday and Anglian Water officials were on site within half an hour.
Anglian Water spokesman Graham Frankland said: "About a bucket load of the sludge, which is a thick organic material, escaped into the watercourse but it was not enough to have caused any environmental impact."
The vandal attack followed a similar incident at 5.15pm on Saturday in which there was no discharge.
Mr Frankland said: "There was no pollution to the river in the first attack. The intruders redirected some of the sewage within the site but there was no discharge into the environment. The system was just recycling within itself."
He said because the system was disturbed, alarms were triggered at the unmanned site but it had not caused any major problems.
The Environment Agency has been informed. Det Sgt Roy Clark, of Colchester police, said: "Someone opened valves and raw sewage escaped into the watercourse. They also smashed up control boxes and managed to get hold of a set of keys which they threw into the tanks."
Anyone with any information is asked to contact police on 01206 762212 or Crimestoppers on 0800 555111
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